The anterior cruciate ligament (ACL) and medial meniscus (MM) are frequently injured knee structures. Losing the functions of either or both of these structures can dramatically alter joint kinematics, causing cartilage damage and the onset of degenerative joint disease and osteoarthritis in the longer term. While investigators are beginning to accurately characterize normal knee kinematics and ligament surface strains in human subjects, critically important tissue forces and contact stresses are impossible to measure in humans without direct calibration. To address this concern, the investigators propose to determine ACL forces and meniscus contact stresses in the goat knee for selected in vivo activities in a robot. We seek to [unreadable] test the global hypothesis that in vivo tissue forces and deformations and tissue-tissue interactions [unreadable] can be predicted by accurately reproducing in vivo knee kinematics in an in vitro setting. To test this global hypothesis, we will examine 5 specific aims: [unreadable] [unreadable] Aim 1: Instrument the goat knee at surgery with ultrasonic crystals and tissue force transducers to precisely monitor relative bone positions and transducer voltages during controlled in vivo activities after surgery. [unreadable] Aim 2: In the laboratory, transform the relative bone positions into joint kinematics to drive a robot to reproduce the knee state during these in vivo activities. Compare actual and simulated joint rotations and translations to determine if in vivo kinematics have been recreated within an acceptable tolerance during the stance phase of gait for each activity. [unreadable] Aim 3: Compare in vivo and in vitro tissue force transducer voltages to determine if in vivo voltages have been recreated within an acceptable tolerance during the stance phase of gait for each activity. [unreadable] Aim 4: Perform selective cutting experiments to study how capsular, ligamentous, and meniscus structures influence the functions of the ACL and menisci for simulated in vivo activities. [unreadable] Aim 5: Calibrate the force transducers to determine ACL forces and meniscus contact stresses for each in activity. Measure the associated tissue deformations for each in vivo activity. [unreadable] [unreadable] This research program will dramatically improve our understanding of normal knee motion and forces. These technologies will serve as a platform for studying injury, repair and reconstruction in the knee and other joints and to develop functional tissue engineering parameters. [unreadable] [unreadable] [unreadable]